In principle, it is known that compact circuit breakers (MCCB=Molded Case Circuit Breakers) are designed in accordance with the principle of the magnetic repulsion of contacts, for example. In this case, the contacts open before the expected peak value of the short-circuit current is reached. Owing to the magnetic repulsion of the contacts, the thermal loading and the mechanical loading as a result of the sudden short-circuit current of the system components which can occur during a short circuit are reduced considerably.
A compact circuit breaker is used, for example, for realizing a dual function, namely protection of a system from overload and short-circuit currents and protection of lines and electrical operating devices from damage as a result of ground faults, for example. In order to protect a system from overload currents or short-circuit currents, the compact circuit breaker, which can also be referred to as thermal-magnetic circuit breaker, has a thermal-magnetic trip unit (TMTU). The thermal-magnetic trip unit has a thermal tripping device in order to protect the electrical circuit or an electrical device from damage owing to an overload, and a magnetic tripping device in order to protect the electrical circuit or an electrical device from damage owing to a short circuit.
A short circuit and in particular an electrical short circuit is generally known as an accidental or unintentionally arising conductive connection between two or more conductive parts and primarily between two nodes of the electrical circuit, as a result of which the electrical potential differences between these conductive parts drop to a value equal to zero or close to zero. In particular in relation to a compact circuit breaker, a short circuit is an abnormal connection between two isolated phases, which are intended to be isolated or insulated from one another. A short circuit results in the presence of an excessive electrical current, namely an overcurrent, which can result in damage, overheating, fire or even an explosion of the electrical circuit and/or the consumer. An overload is a less extreme state in comparison with a short circuit and is rather a longer-term overcurrent state.
The thermal tripping device has, for example, a bimetallic strip including two metal strips which are rolled one on top of the other, which metal strips have different coefficients of thermal expansion. The current of the connected consumer in this case flows via a heating winding and heats up the bimetallic strip, which then bends or curves. Owing to the bending movement of the bimetallic strip, control contacts are actuated, for example, or a latching mechanism of a circuit breaker is unlatched.
The magnetic tripping device or the electromagnetic tripping device is constructed, for example, in such a way that, in the event of the occurrence of a short circuit or a short-circuit current, the current flowing through a coil of the electromagnetic tripping device is so high that, for example, an impact armature or a hinged armature is attracted and, owing to this movement, a latching mechanism of the circuit breaker unlatches without delay. For this purpose, known electromagnetic tripping devices also have a yoke or yoke element, for example, which is arranged along or around an electrical line or a current-conducting path in order to generate a magnetic field in the event of the occurrence of a short-circuit current, which magnetic field advantageously attracts the armature element. The armature element or the armature is held in position in a known manner by a spring and in particular a tension spring, with the result that a movement of the armature element in the direction of the yoke element counter to the tensile force or spring force of the spring can therefore only take place in the event of the occurrence of a defined magnetic field and consequently a corresponding tripping short-circuit current.
Compact circuit breakers are preferably power circuit breakers which can be switched on again after tripping owing to an overload or a short-circuit current. In this case, the tripping devices, i.e. the thermal tripping device and the magnetic tripping device, are advantageously arranged in series.
Depending on the use conditions and/or use location of the compact circuit breaker and in particular taking into consideration different rated currents or tripping characteristics and/or on the basis of the resistance to faulty tripping as a result of transient voltages and the time delay in the presence of a fault current, it is therefore indispensable to calibrate or set the magnetic tripping of the armature so that the armature is attracted only in the case of the occurrence of a defined short-circuit current owing to the magnetic field generated by the yoke element. For this purpose, in a known manner the spring force acting on the armature element is changed, wherein primarily the corresponding spring element, having a corresponding spring force, is mounted. This requires the use of a large number of different spring element types or a large number of springs with a different spring force.
Furthermore, it is considered disadvantageous that the armature element is mounted movably in a known manner by virtue of holding regions or projections of the yoke element, with the result that, in particular in the event of the occurrence of a short-circuit current, more than one magnetic field, and in particular two magnetic fields, are generated by the yoke element, which magnetic fields negatively influence one another, and in particular impair one another, and therefore also negatively influence and in particular impair the movement of the armature element with respect to the yoke element. Therefore, it is conceivable that sufficiently quick tripping of the movement of the armature element for interruption of the current flow in the electrical circuit cannot be realized.